Recently, an optical recording medium, which has a data recording surface having optically changed patterns, such as light and dark pits, formed thereon to represent digital data, has been attracting public attention. The optical recording medium could realize a memory of a large capacity because of its high density data recording capability. Thus, there has been contemplated, as an optical memory, not only a disc type memory but also a card type memory.
In the card type optical memory or optical memory card, recording is made by forming optical (or magnetooptical) changes in response to data to be recorded at discrete spots on the surface thereof, and then a light beam, such as a laser beam, is projected thereonto to read the changes.
More specifically, fine unevenness or light and dark patterns are provided on a surface of a recording medium or card. A light beam is projected onto such optical patterns to read the recorded data according to a difference in reflectivities, refractive indexes, or transmissivities between the optically changed states of the medium. Alternatively, the recording medium may have photothermo-magnetical record thereon and the recorded data may be read depending upon a change of polarization of the projected light due to the magnetooptical effect.
In juxtaposition with each of the data tracks of the optical memory card, there are provided a clock track for synchronization in a data write/read operation and a tracking guide line as a reference for letting the optical beam from an optical reading system follow the track accurately.
The write/read timing for the optical recording medium is conventionally determined by a clock signal derived by a detector dedicated for detecting clock bits of the clock track.
Incidentally, if the clock is not accurately read in reading the clock track of the optical recording medium by reason of dust, scratch or the like on the optical recording medium or fluctuations of the moving velocity of the recording medium relative to the optical reading system, then it will be difficult to achieve accurate write/read operation. An absence of the clock during a short term or in relatively slow fluctuations of the velocity can be coped with by using a phase locked loop (PLL) circuit which produces a clock of a frequency synchronized with the input of the read out clock.
FIG. 5 illustrates a typical arrangement of a prior art PLL circuit. The PLL circuit 66 comprises a phase comparator 52, a low pass filter (LPF) 54, an amplifier 56, a voltage controlled oscillator (VCO) 58 and a 1/N frequency divider 53. This circuit compares, in phase, a received input signal with an output of the divider 53 so as to match the phase through control of the VCO 58, thus producing, at the output thereof, a clock of a frequency of N times of that of the input signal. When N is 1, a VCO output fonformed with the frequency of the input signal will appear at the output terminal of the PLL circuit 66.
However, for a long-term absence of the clock or for an abrupt change of the relative moving velocity of the medium, the PLL circuit could not ensure the accurate data reading and it may fall into an out-of-locked state. In this case, the following problems may occur. When one data is formed of a plurality of words (a "word" used here means one bit or a plurality of bits to be read by one clock), if the PLL circuit gets unlocked or gets into an out-of-locked state, some words will not be read normally. In this state, if the PLL circuit is reinstated into the locked position again, it can in no way be identified which word in the data is being read.